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Scarfed headstock joint question

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Scarfed headstock joint question

Postby Brian Evans » Wed Apr 12, 2017 9:44 am

With a traditional scarfed headstock joint you cut on an angle, flip one piece over and glue the two pieces together. One piece always is glued to side grain, and the other piece is always glued to end grain. Not perfect end grain like a 90 degree cut would give you, but end grain none the less. Why is this OK and what are the strength implications on the joint?

Thanks as always! Brian
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Re: Scarfed headstock joint question

Postby Bryan Bear » Wed Apr 12, 2017 11:45 am

One side is "endgrainish" but as you point out it is not 90 degree endgrain more like 15 degrees which makes a lot of difference. That joint is probably a lot stronger than the risk of breakage due to short grain with an unscarfed neck. I (almost) always have a front and back veneer that helps reinforce the joint and sometimes use long grain ears on the side of the peghead (depending on how I made the neck). With all that going on, I don't worry about the glue joint one bit.
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Re: Scarfed headstock joint question

Postby Beate Ritzert » Wed Apr 12, 2017 12:34 pm

The headstock joint gets additional support from the fingerboard. It should be possible to make this thick and strong, shouldn't it?

BTW: it is also possible to make a 3 piece neck where the central section uses the inverse version of the scarfed headstock (central stripe longer, and headstock glued from below). So the headstock and the neck have an increased glueing surface with nearly parallel grain.
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Re: Scarfed headstock joint question

Postby Rodger Knox » Wed Apr 12, 2017 1:05 pm

There's two ways to do a scarf joint. One has the joint on the neck shaft under fingerboard, the other has it in the middle of the headstock.
I like the first, but the second is easier to hide.
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Re: Scarfed headstock joint question

Postby Alan Carruth » Wed Apr 12, 2017 1:59 pm

I've always made scarf joints with the neck shaft extending into the head, and the added piece on the bottom. With the large glue surface, the low angle, and a head veneer, it's actually pretty strong. I have seen badly done production ones where the scarf itself let go, but the head was still held on by the face veneer. I'm not saying those are easy to tune, mind you, but they do stay together...

At one point I worked on a lot of low-end import solid bodies with the other arrangement: the scarfed piece was glued on top, so there was a segment of the neck that had grain running parallel to the head surface. This meant that the fingerboard was glued to the semi-end-grain. There tended to be a hump in those necks around the area where the scarf ended. I didn't like them.

As Bryan says, a scarf joint with a back strap is just about bullet proof. When I do that I like to extent the strap up the neck for some distance, tapering it out to nothing around the 3d to 5th fret. This is a nice cosmetic effect. If you make the neck with a center stripe, and use the same material for the back strap it looks as though you meant it. I've seen a lot of old banjos done that way. It's especially effective with those V-profile necks. Make the back strap about 2-3mm thick, and bend it to match the headstock angle. Then fit it onto the back of the head and neck. I glue them with Titebond and a caul.
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Re: Scarfed headstock joint question

Postby Brian Evans » Wed Apr 12, 2017 3:47 pm

Alan, with that method you are more scarf jointing the head stock than the neck, would you agree? The joint is not really in the neck shaft or even the angled transition portion, it is extending the "stub" of the headstock. How thick is a typical backstrap and headstock veneer? I normally use a piece about 1/8" thick for my top veneer. If I used a similar thickness for the back, there wouldn't be much headstock left!

Brian
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Re: Scarfed headstock joint question

Postby Alan Carruth » Thu Apr 13, 2017 4:07 pm

Brian Evans wrote:
"Alan, with that method you are more scarf jointing the head stock than the neck, would you agree?"

Certainly. I suppose having the scarf up in the head incurs the problem of weak short grain above the nut, at least in theory. A lot depends on where the scarf is, of course: I've never had problems with it.

1/8" for the face or backstrap veneers is about right, although I'd make them thinner if I used both. You're right that there's not much head left when you do that, but the assembly is certainly strong.

With all of that said, on my own work I pretty much just use a traditional through V-joint; not the modified bridle joint Martin used. IMO this is the best way to put on a head. It takes some skill and a bit of time, to do, of course. My students mostly use either laminated necks or one-piece ones unless they take the plunge and do a V-joint.
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Re: Scarfed headstock joint question

Postby Bryan Bear » Thu Apr 13, 2017 5:04 pm

Alan (and others) may correct my assumptions, but a few of my thoughts. . .

The scarf does end up being in just the peghead if you glue it in that fashion. The thicker the blank, the further out there it will end up once it is all shaped. I like to thin that part of the blank so the scarf is closer to the nut instead of way out in space. My reasoning is that the short grain area is smaller and in an area that is getting less leverage that it would halfway down the peghead. I also like to use a volute which makes part of the short grain area thicker. Also truss rod access through the soundhole.

IMHO, the veneers don't have to be super thick if you are doing the front and the back. My back veneers are nowhere near 1/8" and I still feel like it offers a lot of support to the vulnerable section of the peghead. Drywal is pretty brittle, but holds up surprisingly well with only a paper layer on each side. Score one side of the paper and you can snap it easily. . . I could be wrong, but that is how I see the back veneer. Without it, a sharp hit from the front of the peghead could split the short grain easily just like scoring the back of a sheet of wallboard and hitting it from the front to break it. With even a thin long grain veneer, you get a whole lot of extra support.
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